This invention relates to catalysts useful in catalytic hydrocarbon conversion operations, such as cracking. One type catalyst employs as a catalytic component aluminosilicate zeolite having an X-ray diffraction pattern similar to that of the zeolites with the structure faujasite. (See, for example, the Milton Patent U.S. Pat. No. 2,882,244 and the Breck U.S. Pat. No. 3,130,007). In the catalysts which incorporate such zeolites, the alumino-silicate zeolite is exchanged usually with a polyvalent cation, such as an alkaline earth or a rare-earth cation. In the usual case, such zeolites have been mixed with a matrix. The matrix as used or suggested in the prior art includes alumina, hydrated alumina, such as pseudoboehmite, clays and treated clays, such as acid-treated clays and silica, such as silica gel and colloidal silica sols.
An example of such catalyst is a composite of hydrated alumina, an acid-treated clay, and a material identified as silicated clay in U.S. Pat. No. 3,446,727 which has been exchanged with magnesium and rare-earth cations. The so-called silicated clay may be produced with Al.sub.2 O.sub.3 :SiO.sub.2 ratios of from 1:2 to 1:6. The clay which is converted may be either the acid-treated clay or kaolinite which has been calcined to destroy its X-ray crystallinity. The X-ray pattern of the silicated clay has peaks which are similar to those found in the faujasite X-ray diffraction pattern. All of the foregoing alumino-silicates are herein referred to as zeolites of the faujasite type.
The zeolite catalysts referred to above are unstable when subjected to high temperatures, particularly in the presence of steam. Their catalytic activity is reduced rapidly.
The cracking process operates at two levels of temperature. The catalyst is in contact with hydrocarbon vapor to be converted at a relatively high temperature. In this process, the catalyst becomes contaminated with carbon and high-boiling hydrocarbon. In order to remove the contaminants, the catalyst is subjected to regeneration before it is returned to the cracking zone. The catalyst is purged with steam to remove hydrocarbon vapors and introduced into a regeneration zone where the carbon and hydrocarbons in the catalyst are burned by hot air introduced into the regeneration zone. The regeneration temperature is many degrees higher than the reaction temperature. The regenerated catalyst is returned to the reaction zone. The zeolite component of the prior art catalysts loses a substantial portion of its crystallinity and activity under these conditions.
In order to test the activity of the catalyst, it is the practice in the catalytic-cracking art to measure the catalyst activity by a bench-scale test. Various tests have been accepted by the cracking art. An early and widely used test was the Cat A Test. (See U.S. Pat. No. 3,446,727.)
More recently, an activity test known as the microactivity test has been adopted. (See Oil and Gas Journal, 1966, Vol. 64, No. 39, pp. 7, 84, 85; and Nov. 22, 1971, pp. 60-68.)
When the "high activity" catalysts were originally developed, the cracking and regeneration temperatures were at a level which permitted the use of these catalysts. (See U.S. Pat. No. 3,446,727.) More recently, the regeneration step required higher temperature conditions. These thermal conditions are so severe that a catalyst of improved stability is required. The "high-activity" catalysts of the prior art referred to above are deactivated in a substantial degree when subjected to these more severe conditions.
In order to test the thermal stability of the catalyst and, therefore, its resistance to the high temperature and steam conditions encountered in commercial cracking opertions, it is subjected to high-temperature steam treatment prior to being subjected to the bench-scale tests. Originally, when the catalysts were regenerated under conditions of moderate severity, the catalysts were tested by subjecting them to steam at 1350.degree. F. for 4 hours. (See U.S. Pat. Nos. 2,035,463 and 3,446,727.) Subsequently, the temperature of the steaming was increased to 1450.degree. F. for two hours. This method is referred to in this specification as M Steaming. As the regenerator temperature of the chemical cracking process became more severe, it was found that the temperature of steaming prior to testing should be increased to 1500.degree. F. for 2 hours (S steaming) in order for the bench-scale test to give results which would be commercially meaningful.
To be representative of the still higher regeneration temperatures of modern catalytic cracking units, the steaming conditions are made even more severe. A temperature of 1550.degree. F. for 2 hours (S+ steaming) prior to testing for catalysts was found to be more nearly representative of the effect of the regeneration operations in these more modern cracking operations. This steam pretreatment is referred to in this application as S+ steaming.
As is more fully shown below, zeolite catalysts of the faujasite type when subjected to S+ steaming are substantially inactive.
While we do not wish to be bound by any theory of why this is so, we note that when these catalysts are subjected to heat and steam in regeneration during a commercial cracking operation, their X-ray diffraction pattern shows a substantial modification of the peaks characteristic of faujasite.
The prior art has formulated such catalyst from zeolites, such as sodium zeolites of the faujasite type which have been exchanged with ammonium hydroxide or polyvalent cations, such as magnesium, rare earth or several thereof. Such exchange process is carried out. Such catalysts have been used in cracking of petroleum fractions. These include the so-called fixed-bed systems in which the cracking reaction and the regeneration are carried out in alternate stages without moving the catalyst. One system is the moving-bed type in which the catalyst mass moves continuously in cycles of operation from the reaction zone to the regeneration zone and returns to the reaction zone. Catalysts used for these systems are of substantial size, such as cylindrical pellets of, for example, 3/16 to 1/4 inch length and 3/16 inch in diameter.
A widely used process is the fluid catalytic cracking process. In this process, the catalyst is in the form of fine particles, for example, 20-80 micron diameter in microspheres. These are formed by spray drying water suspensions of the catalyst components. In the fluid catalytic cracking process, these microspheres are suspended in the hydrocarbon vapors in "dense" phase under cracking conditions. The hydrocarbon steam passes to a disengaging zone. Catalysts which are separated from the vapors are returned to the "dense" phase. The separated vapors are passed to the fractionation device. Spent catalyst passes to a steam-stripping section for removal of hydrocarbons. It is then transferred by carrier steam to the regeneration zone. The carbon and hydrocarbon contaminants in the spent catalyst are removed by combustion with hot air. The regenerated catalyst is returned to the reaction zone. The combustion gases are exhausted through a cyclone or electrostatic collectors.
In this process, the catalyst particles encounter excessive abrasion as they collide with each other and with the walls of the apparatus. Excessive loss of catalyst in the effluent gases is encountered. Important, also, from an environmental standpoint is the discharge of particles to the air resulting from an inefficient operation of the catalyst separating devices.
The friability of the catalysts when used in the fluid catalytic operations is a disadvantage. It requires replenishment of the catalyst to make up for the loss due to attrition of the catalyst and causes a variation in the space velocity, and thus in the rate of the conversion.
In order to rate catalysts according to their attrition resistance for use in fluid catalyst cracking process, a bench-scale test has been devised and has been used in this art. It has been accepted by the industry as a suitable measure of the abrasion resistance of spray-dried fluid catalyst particles. This test simulates the fluidcracking process attrition conditions although operating under ordinary room temperature. It measures the rate of weight loss in a sample of microspheres under test which is lost in the effluent gases.